Field of the Invention
The present disclosure relates to a piston for use in a reciprocating compressor such as a reciprocating compressor adapted for use on rail vehicles for the purpose of supplying compressed air to pneumatic units associated with the rail vehicle, and, more particularly, to a piston configured for maximizing the volumetric efficiency of the compressor.
Description of Related Art
Normally, a pneumatic system is provided for a rail vehicle by which the brakes of the rail vehicle are operated. An air compressor is used to supply compressed air to one or more pneumatic units associated with the rail vehicle involved in the operation of the brakes. The air compressor usually consists of a driving unit, such as an electric motor, and of a compressor unit, which typically consists of several piston-cylinder arrangements that are driven by a crankshaft. The crankshaft is driven by the driving unit and includes connecting rods to convert the rotating movement of the driving unit into linear movement for each piston to supply compressed air to the downstream units. Furthermore, air compressor units for use on rail vehicles may have a single-stage or a multi-stage construction, with at least one low-pressure stage and one high-pressure stage.
A reciprocating compressor is the simplest example of the positive displacement class of compressors. This type of compressor was also the earliest designed. Like reciprocating incompressible fluid pumps, reciprocating compressors can also be either single acting or double acting. Single-acting compressors are usually of the trunk type. Double-acting compressors are usually of the crosshead type.
Reciprocating compressors are available in both lubricated and non-lubricated versions. The lubricated versions provide lubrication for the moving pistons (in the cylinder), most often via an oil pump and injection of oil to the cylinder bore. There are some applications where oil must be completely omitted from the compressed air or gas exiting the machine. For such applications where a reciprocating piston type of compressor is required, there are non-liquid lubricated compressors. Recently, dry-running air compressors have found increased usage in the rail vehicle field. A dry-running air compressor operates without lubricating oil situated in the housing and is said to be “oil-free”. These compressors have piston rings around the periphery of each piston for dynamic sealing to allow compression. These piston rings are made of special wear-resistant dry lubricating materials such as polytetrafluorethylene. They also utilize tight tolerances and special coating or rings of similar material composition as the piston rings to guide the piston within the cylinder bore. Trunk type non-lubricated compressors have dry crankcases with permanently lubricated bearings. Crosshead type compressors usually have lengthened piston rods to ensure that no oil wet parts enter the compression space.
The volumetric efficiency of a reciprocating compressor is defined by the ratio of the actual amount of air flow discharged from a compressor (actual air delivery volume) to the total amount of air that can theoretically flow into the compressor inlet during the intake stroke (swept volume). The swept volume is defined by the speed of the compressor, the diameter of the cylinders, and the distance traveled by the cylinders in each rotation (the compressor stroke). The actual amount of compressed air delivered from the compressor is effected by several losses including leakage and piston ring blow by, however a major factor is the amount of volume between the top of the piston (the crown) and the cylinder valves when the piston is at the top of its stoke or top dead center.
This volume is called the clearance volume. The clearance volume defines the amount of air that is pulled into the cylinder on the intake stroke, compressed during the compression stroke, but is not discharged from the cylinder before the next intake stroke. Instead, while fresh atmospheric air enters the cylinder during the intake stroke, the compressed air left behind in the clearance volume re-expands filling the cylinder volume that otherwise would have been filled with atmospheric air being pulled into the cylinder. The end result of the re-expansion is that the cylinder is not capable of delivering, during the discharge stroke, the same amount of air that can be theoretically ingested into the cylinder during the intake stroke.
This same phenomenon occurs within each stage of a multiple stage compressor. The difference is that the inlet air is at a higher pressure than ambient if the cylinder is part of the second, third, or subsequent stage of a compressor. The clearance volumes in each cylinder and stage are additive and result in a reduction in overall compressor efficiency. The role that the clearance volume plays in determining the overall efficiency of an air compressor makes it a critical characteristic of compressor design. Simply stated, a compressor with less clearance volume is more efficient than a compressor with more clearance volume.
In an oil free compressor, it is common practice to utilize piston rings made from composite materials that have self-lubricating properties. A common problem with composite rings is that they have coefficients of expansion that are greater than typical metals. The expansion factor of the rings may be as much as 10-15 times greater than the typical materials used for the piston and cylinder. As a result, the clearances between the piston rings and cylinder wall must be greater than in a compressor with a lubricated cylinder where the piston rings and cylinder liners are made of similar materials. The clearances must account for cylinder temperatures that may be five times greater than ambient conditions and not result in a complete reduction of piston to piston ring to cylinder liner radial clearance. A lack of clearance between the piston, piston rings, and cylinder liner will result in excessive heat generation in the cylinder and eventual piston seizure, causing a compressor failure.
At the same time, excessive clearance between the piston, piston rings, and cylinder liner can result in a piston that slaps in the cylinder bore, especially during the periods of the stroke when the piston is changing directions. This slap results in excessive compressor vibration, piston ring and cylinder liner wear, and premature compressor failure.
As a compressor with an oil free cylinder must have greater piston to cylinder liner clearance, there is a natural tendency for oil free compressors to have greater piston tilt around the wrist pin within a cylinder. In other words, the angle between the top of the piston and cylinder liner is not held at a near perfect 90 degrees for the entire length of the stroke. This is specific to compressor cylinders with trunk type pistons. Pistons of a compressor with a cross head are always held square to the cylinder bore and do not experience the same tilt with the bore. This type of design is more common and is typically for very large reciprocating compressors typically found in stationary industrial type use.
The increased tilt of the piston at the top of the stroke described above must be accounted for when determining the amount of clearance volume in the cylinder. In other words, the clearance volume must be greater to account for the piston tilting when moving from one side of the bore to the other when the piston changes direction at the top of its stroke. This angle means that the distance between the piston crown and cylinder head/valve on one side of the piston is greater than the distance between the piston crown and the cylinder head/valve on the other side of the piston. To avoid contact between the piston and the cylinder head/valve at the top of the stroke, the clearance volume must be increased to account for the piston angle created by the large clearance between the piston and cylinder liner.
The degree of angle of the piston at the top of the stroke is proportional to the size of the piston and the clearance between the piston and cylinder liner. Therefore, during compression start up and operation in cold temperature environments, the piston angle will be greater due to the greater clearance. The greater cold clearance is due to the need to account for the different rates of material expansion and the requirement that a minimum clearance is required across the entire thermal operational range of the compressor. The issue is also compounded by compressors that have an unloading feature. During unloaded operation, the compressor pistons are reciprocating within the cylinders without compressing air. Compressed air acts to push the piston away from the compressor head/valve and hold the piston crown perpendicular in the cylinder liner during the piston stroke. This helps to reduce the amount of piston tilt at the top of the stroke as well as to hold the internal clearances of the parts within the connecting rods and piston assembly to a minimum resulting in the maximum amount of clearance volume at the top of the stroke. When a cylinder is in the unloaded state, there is no force applied to the top of the piston from the compressed gas to hold the internal clearances at a minimum and keep the piston from tilting within the cylinder.